1. Field of the Invention
The present invention relates generally to gas turbine engines and, more particularly, to a lower pressure drop radial inflow air-oil separating arrangement in the engine and an air-oil separator employed in the arrangement.
2. Description of the Prior Art
Gas turbine engines typically include a core engine having a compressor for compressing air entering the core engine, a combustor where fuel is mixed with the compressed air and then burned to create a high energy gas stream, and a first or high pressure turbine which extracts energy from the gas stream to drive the compressor. In aircraft turbofan engines, a second turbine or low pressure turbine located downstream from the core engine extracts more energy from the gas stream for driving a fan. The fan provides the main propulsive thrust generated by the engine.
Bearings are used in the engine to accurately locate and rotatably mount rotors with respect to stators in the compressor and high and low pressure turbines of the engine. The temperature capability of the bearings is, however, quite limited compared to the temperatures of many areas of the engine flowpath through the compressor, combustor and high and low pressure turbines, which flowpath areas are located in close proximity to the bearing. For example, bearings can operate up to 600.degree. F., whereas compressor exit temperature often exceeds 1100.degree. F. and turbine inlet temperature often exceeds 2000.degree. F.
In order to prevent overheating of the bearings, lubricating oil and seals must be provided to prevent the hot air in the engine flowpath from reaching the bearing sumps, and lubricating oil flows must be sufficient to carry away heat generated internally by the bearings because of their high relative speed of rotation. Non-contacting or labyrinth seals are one type of seals that are employed at the sites of many bearings. A labyrinth seal includes one or more pointed teeth usually mounted on a rotating seal member and running in close proximity to a cylindrical or stepped cylindrical stationary stator with air or gas flow between the two members.
Labyrinth seals require air pressurization to prevent leaking of oil through the seals. Pressurization of the seals, in turn, pressurizes the oil sump. However, the sump pressure must be maintained at a proper balance for the lubricating system to function properly. On the one hand, if the sump is over-pressurized, oil will be forced out through the oil seals. On the other hand, if the sump is under-pressurized, the performance of the oil pump of the lubricating system will be adversely affected.
The pressurized air must be vented from the sump in a controlled manner in order to maintain sump pressure at the proper balance. However, the pressurized air is mixed with particles of the oil in the sump. Therefore, the oil must be separated from the air before venting of the air in order to minimize the amount of oil carried overboard by the venting air. An air-oil separator device is typically employed between the oil sump and a center vent passage through the inner drive shaft of the engine to achieve the desired separation. Air-oil separator devices utilized heretofore have several shortcomings which adversely affect seal effectiveness and oil consumption.
One prior art air-oil separator device employs a concentric arrangement of cylindrical plates having staggered air holes which define a tortuous path for the flow of air through the apparatus. This separator device produces a large pressure drop which contributes to the sump pressure being too high and causes oil to backflow the sump labyrinth seals. One reason for the high pressure drop is the free vortex flow of the air once it exits the separator device and travels to the center vent passage. Another reason for the high pressure drop is that the tortuous path of air flow through the separator devices produces a non-determinable pressure drop which cannot be controlled.
Another prior art air-oil separator device employs a series of aligned air flow orifices of different diameters which makes it impossible to predict the pressure drop across the device and thus to determine the efficiency of the separator device. Also, the air flow has a higher pressure drop due to combined forced and free vortex flow.
Consequently, a need exists for improvement of air-oil separation in a manner that will increase separator efficiency and improve sump pressure control.